Catalytic reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably of the platinum type, are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcyclo-paraffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
Platinum has been widely commercially used in recent years in the production of reforming catalysts, and platinum-on-alumina catalysts have been commercially employed in refineries for the last few decades. In the last decade, polymetallic metal catalysts have been employed to provide, at reforming conditions, improved catalyst activity, selectivity and stability. Thus, additional metallic components have been added to platinum as promotors to further improve, particularly, the activity of selectivity, or both, of the basic platinum catalyst, e.g., iridium, rhenium, selenium, tin, and the like. Platinum-iridium catalysts, for example, possess superior activity for use in reforming operations as compared with platinum catalysts. Selenium has also been added to the platinum-iridium catalyst, the catalyst containing the triumvirate of metals possessing admirably higher selectivity as contrasted with platinum catalysts, or platinum-iridium catalysts, respectively, selectivity being defined as the ability of the catalyst to produce high yields of C.sub.5.sup.+ liquid products with concurrent low production of normally gaseous hydrocarbons, i.e., methane and other gaseous hydrocarbons, and coke.
In a typical process, a series of reactors constitute the heart of the reforming unit. Each reforming reactor is generally provided with fixed beds of the catalyst which receive upflow or downflow feed, and each is provided with a preheater or interstage heater, because the reactions which take place are endothermic. A naphtha feed, with hydrogen, or recycle gas, is concurrently passed through a preheat furnace and reactor, and then in sequence through subsequent heaters and reactors of the series. The product from the last reactor is separated into a liquid fraction, i.e., a C.sub.5.sup.+ or a C.sub.5 /430.degree. F. fraction, and a vaporous effluent. The latter is a gas rich in hydrogen, which usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated from the C.sub.5.sup.+ liquid product and recycled to the process to minimize coke production. Hydrogen is produced in net yield.
The activity of the catalyst gradually declines due, at least in part, to the building-up of coke. Coke formation is believed to result from cracking and polymerization reactions; perhaps from the deposition of coke precursors such as anthracene, coronene, ovalene and other condensed ring aromatic molecules on the catalyst, these polymerizing to form coke. During operation, the temperature of the process is gradually raised to compensate for the activity loss caused by coke deposition. Eventually, however, economics dictates the necessity of reactivating the catalyst. Consequently, in all processes of this type the catalyst must necessarily be periodically regenerated by removal of the coke from the catalyst. Typically, in the regeneration, the coke is burned from the catalyst at controlled conditions. In a regeneration of this type, the catalyst is contacted with oxygen at flame front temperatures ranging about 800.degree. F. to about 1050.degree. F., this being generally followed by a secondary burn with increased oxygen concentrations as coke is depleted from the catalyst. Coke has also been removed from catalysts by contact with hydrogen at elevated temperature.
Two major types of reforming are generally commercially practiced in the multi reactor units, both of which necessitate periodic reactivation of the catalyst, the initial sequence of which requires regeneration, i.e., burning the coke from the catalyst. Reactivation of the catalyst is then completed in a sequence of steps wherein the agglomerated metal hydrogenation-dehydrogenation components are atomically redispersed. In the semi-regenerative process, a process of the first type, the entire unit is operated by gradually and progressively increasing the temperature to maintain the activity of the catalyst caused by the coke deposition, until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. In the second, or cyclic type of process, the reactors are individually isolated, or in effect swung out of line by various manifolding arrangements, motor operated valving and the like. The catalyst is regenerated to remove the coke deposits, and the reactivated while the other reactors of the series remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, until it is put back in series.
The ability to regenerate the coked catalyst during operation, as in the operation of cyclic units, means of course, that relatively short cycle lengths are possible. Thus, in the operation of a cyclic unit the reforming operation is conducted at higher severities, i.e., lower pressures, lower hydrogen recycle rates and higher temperatures than in a semi-regenerative operation. In cyclic operations, coke deposits on the catalyst at a much higher rate and hence, typically, the catalyst of a reactor must be regenerated every 60 to 80 hours. In contrast, in the operation of a semi regenerative unit the unit usually remains on stream for several months, typically about six months, or longer. Higher C.sub.5.sup.+ liquid and hydrogen yields are obtained in cyclic operations as contrasted with semi-regenerative operations.
In fluidized bed operations, and in fluidized magnetically stabilized bed operation where the catalyst is made magnetic by the incorporation of a small amount of magnetic material and a magnetic field applied, excellect contacting is achieved between the feed and catalyst. Regeneration is accomplished by transport of the fluidized bed from the reaction zone or vessel into a regeneration zone, or vessel. If desired, the bed can be moved into the regeneration zone, or zones, by appropriate manipulation of the magnetic fields.
Platinum-iridium-selenium catalysts offer significant C.sub.5.sup.+ liquid yield and activity credits relative to the best of present-day commercially used catalysts, and these catalysts offer these advantages at fluidized bed, magnetically stabilized, semi-regenerative and cyclic severities. Like other known iridium-containing catalysts, however, these catalysts are highly sensitive to iridium metal agglomeration in the presence of oxygen at high temperatures. Specially developed techniques must therefore be practiced in the regeneration of such catalysts.